A Wellington investigator's "that's not right" hunch has led to a paedophile being jailed for 40 years and a young boy rescued from months of sexual abuse across three continents.

The boy, who was bought for $10,000 as a young baby by a man and his male partner and then adopted, was not only abused by the pair, but also traded to other paedophiles.

One of his guardians, an Australian citizen, was jailed for 40 years in the United States at the weekend. His American partner has also pleaded guilty and will be sentenced later this year.

The bust started up to 18 months ago when Jon Peacock, the team leader at the Department of Internal Affairs' Censorship Compliance unit, discovered photographs of the boy on the computer of a Wellington man - with no children of his own - in an unrelated investigation.

"I was doing a forensic analysis of a computer and found the photographs. They were not objectionable; they were studio or professional shots of the boy, but the poses and the way he looked into the camera were unusual.

"They just did not sit or feel right, and you think, ‘these might not be innocent photographs . . . these could be living record of a crime'.

"We take a very victim-centric approach here, so my first thought was wanting to check with his parents or guardians to figure out if they knew such photos were being taken."

Further analysis of the photographs and a video revealed a Queensland licence plate on a car, prompting Peacock to alert the Argos Task Force in Australia.

"Through fantastic co-operation between police and customs in different parts of the world, the two men, who had been living in Cairns, were identified."

Further inquiries uncovered evidence of offending and authorities in the US became involved.

The investigation revealed that the two men had bought a baby boy for US$8000 ($10,000), sexually abused the child and traded him to others to abuse in the US, France, Germany and Australia. The two men, who also lived in California, bought the boy - believed to be 18 to 24 months old at the time - in an undisclosed country and used falsified documents in Los Angeles to adopt him.

According to Indiana US Attorney Joe Hogsett, the men subjected the boy to some of the most heinous acts of exploitation his office had seen.

Although the abuse identified centred on 12 to 18 months when the boy was about 6 years old, Mr Peacock had little doubt it had gone on years before that.

"The men were the boy's legal guardians, but it would appear there was a premeditated motive of sexual abuse from the start."

Peacock said he was proud the Censorship Compliance Unit was building an international reputation, having played key roles in several cases to rescue abused children and convict paedophiles.

While "any right-minded person" would be offended by the material they had to view in their investigations, Peacock said the overriding emotion was "immense satisfaction" when they helped bring about a positive outcome.

"Rescuing and protecting children from this vile trade is what it's all about," he said.